We aim to develop a diagnostic center for the detection of thromboembolic diseases using newer isotope-labelling techniques. Some of these techniques such as the use of Cr 51-leukocyte, 99mTc urokinase, whole body photoscanning with I 131 fibrinogen and the 99mTc venogram were developed in our own laboratories. We plan to apply these techniques to those patients with high risk of thromboembolic complications and to those clinically suspected of having deep vein thrombosis, with particular emphasis on those post operative orthopedic cases in the above 60 age group where incidence of pulmonary embolism is high. Three groups of new radiopharamaceutical agents will be used in stages of this program. First, the techniques that have been perfected will be used, I-131 fibrinogen whole body scanning for high risk subjects, and Cr 51-leukocytes for clinically diagnosed pre-formed thrombus. The findings will be validated by both conventional venography and our newly developed technique of 99m Tc venography. With the latter technique there is an additional advantage that pulmonary embolism may be picked up by a concurrent lung scan. Other ancillary techniques such as the Doppler Ultrasound Flow Detection will be used as supplemental diagnostic procedures. Second, we shall continue to perfect techniques currently being developed by us in experimental deep vein thrombi in dogs and primates, using 99m Tc and I-131 streptokinase and 99m Tc urokinase and 99m Tc leukocytes third, other newer radionuclides 114m Indium and other newer carriers for them such as plasminogen will be studied. This study will provide information on the incidence and time of onset of deep vein thrombosis, site of thrombosis and the identification of those cases that may progress to pulmonary embolism. These answers cannot be given by the other conventional isotopic methods now in use because of the inability of these methods to detect thrombi by photoscanning and to detect thrombi in the pelvis, e.g. iliac veins, or ovarian veins. The ability of our proposed method to detect thrombi lying deep in the body render them useful for detection of pulmonary emboli and intracardiac thrombi.